The development of rational and appropriate guidelines for setting precise estimates of nutrient intakes for maintenance of human health requires accurate knowledge of the bioavailability of nutrients in foods and human diets. Except for the case of iron, there is little direct data concerned with the bioavailability of essential nutrients in foods as directly consumed by people. Hence, novel and noninvasive approaches are required to develop a sound data base on the bioavailability of nutrients for humans at various stages of life. In this research proposal a new approach is proposed to assess directly the bioavailability of amino acids in protein foods and to assess the impact of food processing and preparation on amino acid bioavailability. The approach is based on the use of stable (non-radioactive) isotopic-labeled amino acids (with 15N and 2H) and a good grade yeast that has been labeled with 15N. A series of experiments in healthy young adults is outlined, the purpose of which is to define the test conditions necessary to establish an "extrinsictag" method for measuring the availability of amino acids present in protein-containing meals. A successful development of the proposed approach will lead to establishment of a procedure for monitoring the consequences of changes in food production and procedures on the availability of nutritionally essential amino acids in the diet of the U.S. population. It will also facilitate a more rational development of laboratory methods for evaluating the quality of the U.S. food supply as well as provide new important information on the contribution of digestion-absorption and the splanchnic bed (gut and liver) to the overall utilization of exogenous amino acids in man.